The present invention relates to a process and an apparatus for drying, by means of a heated drying gas, a liquid film which is applied to a moving substrate and includes vaporizable solvent components and solid components.
Different conventional drying processes and drying apparatuses are used in the drying of materials of large surface area in web form to which liquid films have been applied. Typical materials for drying are, for example, metal or plastic webs, to which liquid films have been applied, which generally consist of vaporizable solvent components, which are removed from the liquid film during the drying process, and of non-vaporizable components, which remain on the substrate after drying.
The coating lends the surfaces of the substrates special properties, which exist in the form desired for subsequent use only after the drying process. An example is the coating of metal webs with light-sensitive layers, which are made up into printing plates. The coating of metal webs or plastic films with substances in the form of a solvent-containing wet film, referred to hereinafter as liquid film, and the subsequent drying thereof consequently represent an operation which requires special installations in order to ensure the desired product quality of the coatings. An essential process step in this operation is the drying of the liquid film which is the final step in the coating process.
In the drying of liquid films on substrates it is usual that a heated gas, in particular air, flows over the surface of the substrates to remove the solvent components from the layer of film. In so doing the heated gas stream is brought into direct contact with the liquid film, which is applied in a layer of even distribution to the substrate which runs through a drying apparatus. In order to ensure a dried film surface which is free from striae and mottling, i.e., an even distribution of the remaining components, the drying installations are equipped with devices which are intended to accomplish a favorable or even distribution of the airflow above the liquid film. The goal of the drying installations is to achieve an even drying over the entire width of the coated web. Furthermore, known drying installations have devices for minimizing disturbances of the air movements which, partly due to turbulent flow movements, have an adverse affect on the film surface which results in mottling.
According to U.S. Pat. No. 3,012,335, a typical design of such a drying apparatus comprises supplying a gas space immediately above the liquid film to be dried with drier gas from a gas space which contains drier gas and is arranged at a certain distance above the web being coated, by means of a multiplicity of slots, nozzles, holes or porous solid bodies. This involves the continuously coated strip or plates on a circulating conveyor belt being passed through the drying apparatus continuously with solvent vapor given off to the drier air. In this operation, the introduced drier air can be constantly renewed in an open circuit and the air enriched with solvent can be completely evacuated. A recirculated air process with partially-renewed drier air and solvent-enriched air may also be used.
Difficulties in evacuating the drier air from the drying space often arise from the fact that, with longitudinal nozzles, or longitudinal slots, arranged transversely with respect to the web advancing direction, a reduction in the nozzle exit velocity occurs in the middle of nozzle banks of slot-type nozzle driers due to the pressure drop in the laterally flowing evacuated gas and consequently the heat and mass transfer is influenced transversely to the web advancing direction. This results in an overdrying at the edges, which in the case of many coating operations causes undesired structuring effects on the dried films.
Therefore, proposals for optimizing the design of nozzle banks in slot-type nozzle driers, which are intended to ensure a constant heat and mass transfer over the entire web width of a drier, are given in the technical journal "Chemie-Ingenieur-Technik", 42nd year, issue 14 (1970), pages 927-929, 43rd year, issue 8 (1971), pages 516to 519 and 45th year, issue 5 (1973), pages 290 to 294. For the optimization of slot-type nozzle driers, mass transfer measurements in impact flow from slot-type nozzle banks having different nozzle surface areas are correlated empirically taking into consideration a broad range of external influencing variables. The relationship found is used to determine optimum nozzle geometries in relation to the fan output per m.sup.2 of material surface. This relationship shows that a constant heat and mass transfer is achieved over the web width by the nozzle slots having a slot width which increases continuously from the edge of the web towards its middle.
When drying webs of material having a large surface area, it is often required for the heat and mass transfer to be very even over the width of the web, in order to avoid local over-drying and the associated deterioration in quality. In these cases, slot-type nozzle banks in which the slots are arranged transversely to the advancing direction of the web preferably are used. The over-drying at the edges thereby observed in the slot-type nozzle drier having an evacuated gas flow path in the direction of the slots is attributable to the distribution of the exit velocity along the slots. In order to avoid this over-drying at the edges, it follows from this, inter alia, for nozzle driers that the surface area near the substrate for the gas evacuation should be at least 3.5 times the nozzle exit surface area in order to obtain an even drying over the width of the web of material.
The current state of the art is to perform a surface treatment on gas-supported web substrates in suspension driers for plasticsheet or metal strips with the aid of a carrying air nozzle system (Journal "gas warme international", Volume 24(1975), No. 12, pages 527 to 531). In this treatment, the drier air enriched with solvent also is extracted again directly in the nozzle banks, in order to eliminate the undesired transversal flow. This produces so-called nozzle driers or impact-jet driers, in which a particular disadvantage is the stagnation point-like flow of individual nozzles, which has a tendency both with a laminar form of flow and with a turbulent form of flow towards flow-physical instabilities which, in particular, in the case of low-viscosity liquid films, inevitably result in irreversible drying structures.
To avoid stagnation point-like flows in the initial region of the drier apparatus, according to PCT Application W082/03450, the drier air is passed from an ante chamber via suitable inlet openings and flow deflectors into a stabilized intermediate space, from there part of the drier air flows via a porous filter element, arranged in the direct vicinity of the liquid film, on to the web to be dried. Such drying is based on the principle that a weak flow of air which is stabilized but highly enriched with solvent forms between the porous filter element, which acts as a protective shield, and the liquid film to be dried and is constantly renewed by exchange with the residual air flowing away transversely above the porous medium and consequently, on account of the relatively short overall length, a pre-drying of the liquid film with a reduced tendency towards the appearance of mottling effects is achieved.
This type of drying is characterized by predominate diffusion of the solvent vapor/air mixture through the porous protective shield, in which, with virtually no convective evacuation at all within the space between strip and protective shield, a complete drying of the liquid film is only possible if the driers are very long or if downstream auxiliary driers are added.